Cream together 1 cup of softened unsalted butter and 1 cup of powdered sugar until light and fluffy, about 2-3 minutes.

Incorporate 1 teaspoon of vanilla extract into the creamed mixture.

Sift together 2 cups of all-purpose flour, 1/4 cup of cornstarch, and 1/2 teaspoon of salt into the bowl.

Divide the dough into two halves; mix 1/4 cup of cocoa powder and 1-2 tablespoons of milk into one half.

Roll out both the vanilla and chocolate dough into equal-sized rectangles, about 1/4 inch thick.

Place the chocolate dough on top of the vanilla dough; roll tightly into a log.

Wrap the dough log in plastic wrap and chill in the refrigerator for at least 1 hour.

Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

Slice the chilled dough log into 1/4-inch thick rounds and place them on the baking sheet.

Bake the cookies for 10-12 minutes, until the edges are lightly golden.